Method and composition for in vivo radiolabeling of red blood cells with 99m Tc

ABSTRACT

A composition containing stannous chloride and potassium perchlorate is suitable for oral administration to a patient for tinning red blood cells of the patient in vivo prior to labeling the red blood cells with radioactive  99m  Tc. The composition is useful for preparing a patient for blood pool imaging, and especially for carrying out equilibrium gated cardiac imaging.

BACKGROUND OF THE INVENTION

This invention relates to radiopharmaceuticals and their use in nuclearmedicine. More particularly, this invention relates to a method andcomposition for labeling red blood cells (RBC) by orally administering astannous-containing compound to a patient to tin the cells in vivofollowed by labeling the RBC with radioactive ^(99m) Tc. This inventionis particularly applicable to tests designed to image blood pools in apatient, and especially for function studies involving the heart using^(99m) Tc-labeled red blood cells as the radiopharmaceutical.

Radiopharmaceuticals approved for general distribution and in vivodiagnosis can be used for imaging, organ flow studies, organ functionstudies, organ localization and dilution and excretion tests.Radiolabeling of red blood cells and their clinical and researchapplications in nuclear medicine constitute areas of continued interestand steady growth. Significant advances have been made so that at thepresent time radiolabels with sufficient in vitro and in vivo stabilityare available for diverse applications. ^(99m) Tc-labeled red bloodcells have revolutionized the field of cardiovascular nuclear medicineby making possible the imaging of blood pools and the non-invasiveevaluation of various heart parameters with minimum radiation dose andtrauma to the patient. ^(99m) Tc-RBC are also used for detectingvascular malformations.

It is thought that technetium in the form of technetium pertechnetatemoves in and out of the red blood cell but is not bound firmly to thecell in this chemical form. Reduced technetium, on the other hand, doesnot generally cross the cell membrane but does bind irreversibly withhemoglobin or other red cell components. It is also thought that bindingcan be achieved by reduction of the pertechnetate once within the cell,and stannous (Sn²⁺) compounds are the most widely used reducing agentsfor this purpose. In most of the current procedures, red blood cells,generally in whole blood, are contacted with stannous ions using asuitable tin (II) preparation, such as pyrophosphate, glucoheptonate,DTPA, citrate or chloride. This procedure is known as "tinning" the redblood cells and can be carried out in vitro or in vivo.

Protocols for labeling red blood cells with ^(99m) Tc by in vitromethods typically involve withdrawing a blood sample from a patient,incubating the sample with a tin-containing composition, separating theserum by centrifugation, mixing a portion of the RBC with ^(99m) Tcpertechnetate, incubating the RBC and injecting the resulting ^(99m) Tclabeled RBC into the patient. The in vitro methods are characterized byseveral disadvantages. They require centrifugation to separate theplasma, involve multiple transfers of red blood cells, include a numberof handling steps and require multiple venous punctures with theconcomitant risks of tissue and vascular damage, infection anddiscomfort to the patient. In vitro methods frequently require theservices of a skilled technician.

In vivo methods for labeling red blood cells with ^(99m) Tc are based onthe intravenous administration of a suitable amount of stannous ion to apatient followed by an in vivo incubation period, typically about 30minutes. This is followed by the intravenous injection of ^(99m) Tcpertechnetate. While RBC labeling occurs almost immediately, the processrequires two injections. Once again, multiple venous punctures increasethe discomfort to the patient and raise the risk of infection and damageto the biologic systems.

Combination of the in vivo and in vitro methods has been proposed. Thecombined method is essentially an in vivo "tinning" procedure, wherein(Sn II) is intravenously injected into the patient, followed bywithdrawal of a blood sample, incubation of the sample with ^(99m)Tc-pertechnetate to label red blood cells in vitro and reinjection ofthe resulting labelled RBC in plasma to the patient. While the combinedin vivo-in vitro method is said to improve labelling efficiency and thesubsequent imaging process, the method suffers some of the samedisadvantages previously described for the in vivo method, the mostserious being the requirement for multiple intravenous punctures and thecomplexity of the various manipulations required.

The in vitro, in vivo and combined in vitro-in vivo methods for labelingred blood cells require costly stannous chloride solutions havinglimited shelf-life. The existing methods also require costly andrelatively complicated equipment to successfully label RBC for clinicalstudies.

Accordingly, there exists a need in the art for a simple, less costlyand effective method and composition for radiolabelling red blood cellswith ^(99m) Tc. The method should not require a centrifugation step forseparating plasma from RBC. The method should be conducted in vivo tominimize technician time and eliminate the multiplicity ofsample-handling steps required in the in vitro methods. The need forcostly and complicated equipment should be minimized. In addition, therequirement for venous punctures should be reduced in order to minimizetissue and vascular damage in the patient and to lessen patientdiscomfort and the risk of infection. The method and composition shouldprovide high uptake of ^(99m) Tc by red blood cells and be suitable forcarrying out blood pool imaging studies. There should be minimal depositof the radionuclide in background tissue that may obscure definition ofa targeted blood pool organ, such as the heart. The composition shouldbe stable and have a longer shelf-life than stannous chloridepreparations for intravenous use.

SUMMARY OF THE INVENTION

Accordingly, this invention aids in fulfilling these needs in the art byproviding a drug for use in an improved method for radiolabeling redblood cells and especially for use in carrying out blood pool imagingstudies in a patient. In particular, this invention provides acomposition consisting essentially of

(1) about 10 mg to about 30 mg stannous chloride, and

(2) about 250 mg to about 1000 mg potassium perchlorate. The compositioncan optionally include a pharmaceutically acceptable carrier and can bepackaged in the form of a kit.

This invention also provides a method of treating a patient for tinningred blood cells in vivo. The method comprises orally administering tothe patient about 3.7 mg/kg body weight to about 42 mg/kg body weight ofthe composition of the invention and waiting for a time sufficient forSn²⁺ to be absorbed by red blood cells.

In addition, a method is provided for radiolabeling red blood cells of apatient having RBC tinned in accordance with the invention, wherein thepatient's tinned red blood cells are contacted with ^(99m) Tc and thecells are incubated for a time sufficient to radiolabel the cells.

There is also provided a method of carrying out equilibrium blood poolimaging in a patient having red blood cells labeled according to themethod of the invention.

The stannous-containing composition of this invention is intended fororal administration to the patient. Patient treatment is therebysimplified and less time consuming than the intravenous mode ofadministration previously used. Demands for technician time and theassociated expense are thus reduced. Oral administration also increasespatient acceptance and reduces patient discomfort, the risk ofextravasation and the risk of infection when compared with theintravenous administration of stannous compounds heretofore used. Thisinvention additionally improves imaging by improvingtarget-to-background ratio, which makes it possible to reduce radiationdose, expense of the radiopharmaceutical used and potential radiationhazards.

DETAILED DESCRIPTION

The composition of this invention is suitable for "tinning" red bloodcells of the patient in vivo prior to labeling the RBC with radioactive^(99m) Tc. The composition of this invention contains stannous chlorideand potassium perchlorate for this purpose.

Stannous chloride can be employed in any water soluble, non-toxic form.For example, the composition can include SnCl₂ or SnCl₂.2H₂ O. Tinchloride dihydrate (i.e., SnCl₂.2H₂ O) is preferred because it can bereadily formulated in solid dosage form to provide a drug having goodshelf-life. In addition, the dihydrate is believed to be less irritatingto the gastrointenstinal tract than anhydrous stannous chloride. Thestannous chloride should preferably be of pharmaceutical quality.

Oral administration of stannous chloride alone is unsatisfactory becauseof high gastric uptake of Sn²⁺. Administration of ^(99m) Tc after oraladministration of Sn²⁺ results in unacceptably high levels of theradiolabel in the gastrointestinal tract. This occurs even if theradiolabel is intravenously administered to the patient after oraladministration of the stannous chloride. Deposition of the radionuclidein the gut, which constitutes the background when imaging the heart andother vital organs, obscures definition of the targeted blood poolorgan. For example, in blood pool imaging of the heart, this unwantedimaging of the gut results in overlap of the stomach with the heartthereby obscuring heart borders. In addition, there is unwanted loss ofthe radionuclide into other organs that are not of clinical interest.Accordingly, it has been discovered that the composition of theinvention must also contain potassium perchlorate (KClO₄). The potassiumperchlorate should preferably be of pharmaceutical quality.

The composition of this invention consists essentially of about 10 mg toabout 30 mg of the stannous chloride and about 250 mg to 1000 mgpotassium perchlorate. Stannous chloride is employed in an amountsufficient for tinning red blood cells of a patient in vivo prior totagging the RBC with the ^(99m) Tc radiopharmaceutical so that RBC willbe radiolabeled in quantity adequate to carry out a desired clinical ordiagnostic test on the patient. The stannous chloride should be employedin the minimum amount necessary for this purpose. Potassium perchlorateis employed in an amount sufficient to substantially inhibit absorptionof intravenously administered ^(99m) Tc by the gut, while permitting thetinning of RBC and radiolabeling the RBC with the ^(99m) Tc. Typically,the stannous chloride and potassium perchlorate will be employed in thecomposition in a weight ratio of about 1:5 to about 1:100, preferablyabout 1:24 or a range of 1:20 to 1:30.

A preferred composition for tinning red blood cells in vivo inpreparation for equilibrium gated blood pool imaging of the heart in anadult patient of 50 kg or more in body weight consists essentially of 25mg SnCl₂.2H₂ O (±1 mg) and about 600 mg KClO₄ in powder form in agelatin capsule.

The composition of the invention can be used in mammalian therapy. Themammals that can be treated include humans, as well as animals, such asfelines, such as domestic cats, and dogs, guinea pigs, mice, rats,monkeys, pigs, horses and cows. The composition of the invention can beorally administered to the patient either fasting or post-prandially.The dose of the composition administered to the patient is about 8.9mg/kg body weight to about 42 mg/kg body weight. A preferred dosage isabout 8.9 mg/kg to about 10 mg/kg. It will be understood that the dosagewill depend upon the nature of the study being conducted, patient size,whether the patient is in fasting or non-fasting state and the rate ofabsorption by the biologic system. Small dosages are typically employedfor small animals and larger doses for humans. Dosages equal to aboutone-half a normal adult dose can be employed in pediatric applications.Generally, for carrying out equilibrium gated blood pool imaging of theheart, the composition of this invention can be orally administered to afasting adult patient of 50 kg or more in an amount of about 8.9 mg/kgbody weight to about 15 mg/kg body weight, preferably about 8 mg toabout 10 mg/kg body weight. There is no known interaction of thecomposition of the invention with other drugs a patient may take.

The stannous chloride and potassium perchlorate can be combined andtogether orally administered to the patient or each compound can beseparately administered. Preferably, the stannous chloride and potassiumperchlorate are administered so that the compounds can be consumed in asingle swallow by the patient. In some circumstances it may be desirableto orally administer the potassium perchlorate in a form and underconditions that will permit the potassium perchlorate to line walls ofthe gut before stannous chloride contacts the walls. This embodiment canbe carried out in different ways. For example, the potassium perchloratecan be first orally administered to the patient, and after a brief timeinterval, the stannous chloride can be orally administered. As anotherexample, the potassium perchlorate and stannous chloride can beadministered together with the stannous chloride in a timed release formthat will allow the potassium perchlorate to first line the stomach.

The composition and each of the ingredients therein can each be in theform of pills, tablets, capsules or other ingestable forms. Thecomposition and each of its ingredients can each also take the form ofpowders or sustained-release formulations and the like. The compositionof the invention and its ingredients can each be prepared in the form ofsolutions, suspensions, or emulsions in vehicles conventionally employedfor pharmaceutical purposes.

The composition of this invention and each active ingredient can becombined with solid or liquid pharmaceutically acceptable non-toxiccarriers, diluents and adjuvants in order to prepare the compositionsfor use in the treatment of mammals. Such pharmaceutical carriers can besterile liquids, such as water and oils, including those of petroleum,animal, vegetable or synthetic origin. Typical carriers are peanut oil,soybean oil, mineral oil, sesame oil and the like. Water is a preferredcarrier. Saline solutions and aqueous dextrose and glycerol solutionscan also be employed as liquid carriers. Suitable pharmaceuticalexcipients include starch, glucose, lactose, sucrose, gelatine, malt,rice, flour, chalk, silica gel, magnesium carbonate, magnesium stearate,sodium stearate, glycerol monostearate, talc, sodium chloride, driedskim milk, glycerol, propylene glycol, water, ethanol and the like.Other suitable pharmaceutical carriers are described in "Remington'sPharmaceutical Sciences" by E. W. Martin. The compositions will containan effective amount of the active compounds together with a suitableamount of carrier so as to provide the form for proper administration tothe host.

The composition of this invention can be packaged in the form of a kitfor convenience of availability and ease of use in tinning red bloodcells of a patient in preparation for radiolabeling the cells with^(99m) Tc. The kit comprises

(1) a solid, unit dose of stannous compound consisting essentially ofabout 10 mg to about 30 mg of stannous chloride sufficient to tin thered blood cells in vivo; and

(2) a solid, unit dose of potassium perchlorate in an amount of about250 mg to about 1000 mg sufficient to substantially inhibit absorptionof ^(99m) Tc by the gut of the patient, while permitting the tinning ofRBC and radiolabeling of RBC with ^(99m) Tc.

The unit doses can conveniently be in the form of tablets or capsules.The unit dosages can be individually sealed in separate compartments,such as a blister package, and easily removed one-by-one from thepackage. The dose of stannous chloride and the dose of potassiumperchlorate can be combined to form a single, solid dose. The kit caninclude directions for use of the drug contained in the kit.

This invention is carried out with a naturally occurring radionuclide asthe radiopharmaceutical. The radionuclide is ^(99m) Tc, that istechnetium in a metastable state formed by the isomeric transition ofmolybdenum 99. The ^(99m) Tc must be in a water soluble form and shouldpreferably be of pharmaceutical quality.

The ^(99m) Tc radiopharmaceutical is commercially available in preparedform, such as in multiple-dose vials. While convenience and low cost areadvantages realized by the use of these vials, a major drawback is thatthe user must mathematically allow for decay and then calculate thevolume necessary to ensure withdrawal of the desired amount of activity.When the radiopharmaceutical is received in prepared form, the properdosage can be determined from the total volume of the liquid at time ofdelivery, total activity, concentration, date and time of assay orcalibration and the specific activity. A suitable source of ^(99m) Tc isavailable under the trade name Technekow from Mallinkrodt Chemical Co.,of St. Louis, Mo.

Since ^(99m) Tc has a relatively short radioactive half-life of only6.03 hours, the radionuclide can be obtained from a source not ready forimmediate use. A radionuclide generator is suitable for this purpose,and ^(99m) Tc generators are commercially available. The relativelylong-lived ⁹⁹ Mo continually produces through radioactive decay theshorter-lived daughter nuclide ^(99m) Tc. The parent nuclide is firmlyaffixed to a support in the generator and the daughter nuclide isseparated by elution. Ion exchange media in the generator facilitatethis process. Other elution processes, such as distillation, solventextraction and precipitation can also be employed, but are generallymore complex than elution from ion exchange media. The product should beassayed for concentration of daughter nuclide.

Quality assurance testing of the radiolabel should be performed on aregular basis. Molybdenum break-through tests should be performed on allgenerator eluents before administration to the human patient. Aluminumbreak-through can also be performed to ensure radiochemical purity.Thin-layer chromatography or paper chromatography techniques can be usedto identify radiochemical contamination that may be present in theradiopharmaceutical. Chromatography kits for evaluating technetiumpreparations and containing all materials necessary are commerciallyavailable. Ideally, chromatography should be performed beforeadministration of the radiopharmaceutical to the patient. The sterilityand apyrogenicity of the ^(99m) Tc must be assured. Aseptic techniquesmust be employed for preparation and handling of the radionuclide.

Technetium as pertechnetate can be employed in the invention in the formof a water soluble salt with a non-toxic, pharmaceutically acceptablecation. The preferred source of radionuclide for use in the invention issodium pertechnetate (Na ^(99m) TcO₄), procured either in prepackagedform or from a ⁹⁹ Mo/^(99m) Tc generator system. When the salt isdissolved in body fluids, a solution is formed in which sodium ions and^(99m) TcO₄ - are produced.

The clinically appropriate dose administered to the patient and itsconsequent radiation dose to vital organs must be determined. The dosecan be varied depending upon biologic parameters, such as uptake,distribution, retention and release of the radiopharmaceutical in thebody and the fraction of the emitted energy that is absorbed by thetarget. Precaution should be taken to ensure that the patient does notexceed the maximum permissible dose of radiation for the patient's agegroup. The dose will also depend upon the nature of the test to beconducted. The amount of ^(99m) Tc administered in practicing thisinvention is typically about 0.2 mCi per kg to about 0.43 mCi/kg of thepatient's body weight, preferably about 0.2 mCi/kg to about 0.29 mCi/kgbody weight. For conducting an equilibrium gated blood pool imagingstudy of the heart, a dosage of about 0.25 mCi per kg body weight hasbeen found to be suitable.

The radionuclide can assume various forms. The radionuclide can be inthe form of a liquid containing a known concentration of ^(99m) Tc, theactivity of the dose depending on the time of administration and thevolume used. A liquid ^(99m) Tc-containing composition is normallycompounded with water or normal saline solution as the vehicle. Theagent can be a true solution of soluble compounds or complexes, or theremay be colloidal or suspended particles of varying sizes.

The process of adherence of the radiolabel to the red blood cell is timedependent; adherence is a function of the length of time Sn²⁺ ion is inthe milieu of the cell and the time of incubation of the ^(99m) Tcradiolabel with the cell. Incubation of stannous ion in vivo is carriedout for a time sufficient for stannous ion in the composition of theinvention to tin red blood cells in the patient and accumulate in thepatient's blood pool.

The resulting tinned red blood cells are incubated with ^(99m) Tc for atime sufficient for the radiolabel to bind to red blood cells, whichthen accumulate in the blood pool without substantial binding ofradioactive technetium to cells in the gut of the patient. If incubationis carried out in vivo, the ^(99m) Tc must be administered before thereis appreciable clearance of stannous ion from the biologic system. Whilethe ^(99m) Tc can be intravenously administered to the patient when thestannous compound is orally administered, this normally results in lossof the isotope, which is cleared by the kidney while RBC are beingtagged. For this reason, incubation of the tinned cells with the ^(99m)Tc radiolabel is generally commenced about 10 to about 120 minutes,preferably about 20 to about 60 minutes, after oral administration ofthe SnCl₂. An incubation time of about 30 minutes following oral dosingof the stannous compound has been found to be sufficient beforeadministration of ^(99m) Tc in preparation for equilibrium gated bloodpool imaging of the heart.

In the preferred method of this invention, the ^(99m) Tc isintravenously administered to the patient following oral administrationof the composition of the invention. A high percentage of red bloodcells are thus radiolabelled.

While intravenous administration of ^(99m) Tc is preferred, it will beunderstood that the red blood cells tinned in vivo according to theinvention can also be incubated with ^(99m) Tc by the known in vitrotechniques or the modified in vivo/in vitro techniques. For example, thecomposition of this invention can be orally administered to a patient.After incubating in vivo for a time sufficient to tin red blood cells,such as 30 minutes, a blood sample is drawn and anticoagulated bythorough mixing with heparin (10 ml) in a syringe. ^(99m) Tc isincubated in the syringe of blood for 10 to 20 minutes, and then theblood sample is reinjected into the patient. Equilibrium blood poolimaging can then be carried out. In any event, oral administration ofthe composition of this invention replaces the intravenousadministration of SnCl₂ required in prior techniques.

The composition and method of this invention make it possible toefficiently and safely radiolabel red blood cells. The efficiency ofradionuclide uptake by red blood cells compared to plasma (i.e."labeling yield") is at least about 85% and generally at least about 90%in this invention. Preferably, labeling yield is about 95% to about 97%or more. When the labeling yield is at least about 85%, there isadequate delineation of targeted blood pool organs for imaging.

Labeling yield is determined by assaying a sample according to thefollowing technique. A 10 ml sample of whole blood is withdrawn from thepatient and the sample is immediately centrifuged at 800 g for 7minutes. Plasma is separated from blood. A sufficient amount of normalsaline is added to the sample to give a total volume of 8 ml. In thesame geometric tube, each fraction is assayed in a gamma countersubtracting background counts from each. Hematocrit is measured.Labeling yield can then be calculated using the following equation:##EQU1## Radioactivity is determined as of 45 minutes following dosingof the ^(99m) Tc. Measurements on samples after longer lag times can beadjusted for radioactive decay. Studies on canines have shown that redblood cells can be tagged using the composition and method of thisinvention with a labeling yield of about 95 to about 98%, which is equalto the efficiency obtained with in vitro and in vivo methods.

Studies on canines have also demonstrated that stannous chloride in thedosage of this invention can be safely administered and that there israpid clearance of tin from the circulatory system. For example, 1000 mgof stannous chloride dihydrate were administered to canines having bodyweights of 12 to 15 kg. A blood sample was drawn from each host one hourafter administration. The serum was assayed for tin content by atomicspectroscopy with a sensitivity of 0.1 microgram per deciliter. Tin wasundetectable indicating rapid clearance from the circulatory system evenat very high dosages.

The composition of this invention makes it possible to reduce the doseof the radionuclide to the patient. Because there is a reduction in theloss of the radiolabel by uptake in non-targeted organs, a smaller doseof ^(99m) Tc can be employed and the teratogenic effects from radiationexposure possibly reduced.

This invention can be employed for any in vivo radionuclide diagnosticor therapeutic method requiring the presence or use of ^(99m) Tc labeledred blood cells. This invention is preferably employed for radiolabelingred blood cells with ^(99m) Tc in preparation for equilibrium blood poolimaging of biologic systems other than the gut. For example, thisinvention is useful for imaging the heart and other vital organs in apatient. The invention can be employed for blood pool visualization ofspleen, heart, great vessels, aortic bifurcation, testicles and lowerextremeties.

This invention is especially useful for conducting an equilibriummultiple gated blood pool imaging study, which is a widely used methodfor the evaluation of right and left ventricular size and function.Imaging of the cardiac blood pool can be carried out with a restingpatient to assess the ventricular ejection fractions, ventricularvolumes and regional kinesis of the myocardium. It is also possible toestablish the damage done by myocardial infarction and to assess theloss of contractility of the myocardium. Resting patients with valvulardisease or other myocardiopathies can also be studied.

The gated equilbrium cardiac blood pool study can also be carried out onpatients during or after exercise. Studies of this type are useful inpopulation screening for coronary artery disease, for evaluatingpatients with mitral or aortic valve disease and to study myocardiumfunction. By administering oral stannous chloride and oral potassiumperchlorate to a patient the problem of unwanted uptake of ^(99m) Tc inthe gut is overcome and this resolves the subsequent problem of overlapbetween the stomach and the blood pool of vital organs being imaged evenwhen the ^(99m) Tc is intravenously administered.

The blood pool study can be carried out with conventional equipment. Ascintillation detector can be used to show radionuclide distribution inthe body and in its organs, both as a planar imaging system and atomographic system. Both types of systems can utilize stationary ormoving detectors. The imaging devices require collimators and utilize acrystal-photomultiplier assembly. Planar systems can be based on arectilinear scanner, such as a single detector assembly or multipledetector assembly, or a stationary camera, such as a single crystalcamera (scintillation camera or Anger camera) or a multi-crystalstationary imaging device. Scintillation cameras yield X and Y positionsignals that require ancillary data presentation equipment, such as acathode ray tube oscilloscope, a multi-format programmer, a variablepersistence scope or a whole-body imaging table.

Equilibrium gated cardiac imaging can be carried out using conventionaltechniques. Cardiac measurements require the combination of ascintillation camera and a properly programmed computer. Red blood cellsin the patient are tinned using the composition of this invention aspreviously described. ^(99m) Tc pertechnetate is incubated with thetinned RBC. The initial passage of the tracer through the heart isimaged. Scintographic counts are acquired throughout the cardiac cycleand analyzed in short time intervals. As a general routine in restingstudies, a septal left anterior oblique projection and an anteriorprojection are obtained. The study can be carried out during exercise byplacing the patient in a supine position beneath the scintillationcamera. Base-line resting studies are recorded. The patient is allowedto begin exercising and to stabilize. Data is recorded for a period oftime. The exercise load is increased, and once again the patient isallowed a period to stabilize followed by intervals for recording data.Sequential sets of images can be obtained until the patient reaches amaximum exercise level. The equilibrium gated blood pool imaging studycarried out according to this invention produces highly resolved images.

The time of imaging is ideally chosen when the target-to-non-targetratio of radiopharmaceutical is at its maximum. To wait longer wouldserve no purpose because the best target-to-non-target ratio has alreadybeen reached by this time. Delay only results in loss of count rate as aresult of decay and biologic clearance. For example, equilibrium gatedblood pool imaging is generally initiated about 10 minutes to about 60minutes following pertechnetate dosing. The accumulation of radiolabeledred blood cells in the heart appears to reach its maximum at about 30minutes after pertechnetate dosing. Imaging can be carried out over aprolonged time period, if necessary, and for as long as the decay of theradiolabel makes it possible. For instance, equilibrium gated blood poolimaging is generally carried out for about 4 to about 10 minutes. Repeatstudies can be performed serially.

This invention will be more fully understood by reference to thefollowing Examples.

EXAMPLE 1

Three mongrel dogs weighing an average of 16 kg each were selected forstudy. To each dog there was administered 25 mg of hydrous stannouschloride (SnCl₂.2H₂ O; ACS Grade; Aldrich Chemical Co.) and 600 mgpotassium perchlorate (KClO₄ ; ACS Grade; Mallinkrodt) peroral. Thirtyminutes later each animal received ^(99m) Tc (Technekow; Mallinkrodt) in0.9 normal saline solution administered intravenously via the brachialvein. A blood sample (10 ml) was withdrawn from each animal at 10 and 60minutes to determine degree of red cell labeling. Plasma was isolatedfrom each sample by centrifugation and radioactivity of the plasmafraction was compared with radioactivity of the red blood cell fractionby measurement in a well counter. Red cell labeling yield was 90 to 95%at 10 minutes and 95 to 98% at 60 minutes.

Blood pool cardiac images were compared with gastric uptake. Gastricuptake of the radiopharmaceutical was negligible. Backgroundinterference was also negligible.

EXAMPLE 2

Fifteen randomly selected human patients were referred for study forgated blood pool imaging of the heart. Patients were studied in anon-fasting state. Patient size varied from 69.1-95 kg. in weight.

Each patient received a composition containing 25 mg of stannouschloride dihydrate (ACS Grade; Aldrich Chemical Co.) and 600 mg ofpotassium perchlorate (ACS Grade; Mallinkrodt) peroral in a singleswallow. Thirty minutes later, each patient received 0.25 mCi/kg of^(99m) Tc as sodium pertechnetate in aqueous solution administeredintravenously via peripheral upper extremity vein. Ten milliliteraliquots of whole blood were collected from each patient innon-anticoagulant tubes at 10 and 60 minutes after administration of theradiolabel. Each aliquot was immediately centrifuged to separate plasmafrom red blood cells. Radioactivity of red blood cells and plasma wasdetermined in a well counter and efficiency of red blood cell labelingcalculated for the 10 and 60 minute samples.

Views of the heart were obtained with adequate sampling over gastric andspleen regions at 10 to 60 minutes after ^(99m) Tc injection. Staticviews of the region of the salivary and thyroid glands were obtained forqualitative comparison of uptake.

The efficiency of the label was 82 to 99% at the 10 minute interval witha mean of 94%. The efficiency of the label at 60 minutes was 92 to 100%with a mean of 97%.

Gastric uptake was judged to be 0-1+ on an objective scale of 0 to 4+,wherein 0 signifies no uptake in the region, 1+ signifies some uptakeand 4+ signifies complete visualization of borders. Sample size was toosmall for comparison of fasting to non-fasting states. Qualitatively,there was little variation in gastric uptake when compared with thecustomary in vivo labeling with intravenous stannous chloride. Thyroidand salivary gland uptake, however, was qualitatively estimated to beapproximately 1/3 to 1/2 that of the customary in vivo labelingtechniques with intravenous stannous chloride.

Patients tolerated the oral administration of the composition well.There were no complaints of gastric upset or evidence of gastricirritation following administration.

In summary, this invention provides a simple, less costly and effectivemethod for radiolabeling red blood cells with ^(99m) Tc. The method doesnot require a centrifugation step for separating plasma from RBC. Themethod can be conducted in vivo to minimize technician time andeliminate the multiplicity of sample-handling steps required in the invitro methods. The need for costly and complicated equipment isminimized. In addition, the requirement for venous punctures is reducedthereby miminizing tissue and vascular damage and discomfort in thepatient and lessening the risk of infection. The method and compositionprovide high uptake of ^(99m) Tc by red blood cells, and are suitablefor carrying out blood pool imaging studies. There is minimal deposit ofthe radionuclide in background tissue that may obscure definition of atargeted blood pool organ, such as the heart. The composition is stableand has a longer shelf-life than stannous chloride preparations forintravenous use.

I claim:
 1. A method of treating a mammalian patient for tinning red blood cells in vivo, said method comprisingorally administering to the patient about 8.9 mg/kg body weight to about 10 mg/kg body weight of a composition consisting essentially of (1) about 10 mg to about 30 mg stannous chloride, and (2) about 250 mg to about 1000 mg potassium perchlorate; and incubating red blood cells in vivo for a time sufficient for Sn²⁺ to be absorbed by the red blood cells.
 2. A method according to claim 1, wherein the stannous chloride and potassium perchlorate are in a weight ratio of about 1:20 to about 1:30.
 3. Method according to claim 1, wherein the stannous chloride and potassium perchlorate are in a weight ratio of about 1:24.
 4. Method according to claim 1, wherein the red blood cells are incubated in vivo with Sn²⁺ for about 10 minutes to about 120 minutes.
 5. Method according to claim 1, wherein the patient is a human.
 6. Method according to claim 1, wherein the mammal is a canine or feline.
 7. A method for radiolabeling red blood cells of a patient, said method comprisingorally administering to the patient about 8.9 mg/kg body weight to about 42 mg/kg body weight of a composition consisting essentially of (1) about 10 mg to about 30 mg stannous chloride, and (2) about 250 mg to about 1000 mg potassium perchlorate; incubating red blood cells in vivo in the presence of Sn²⁺ for a time sufficient for tin to be absorbed by the red blood cells in the patient; contacting the resulting tinned red blood cells with ^(99m) Tc; and incubating the tinned red blood cells with the ^(99m) Tc for a time sufficient to radiolabel the cells.
 8. Method according to claim 7, wherein the ^(99m) Tc is intravenously administered to the patient.
 9. Method according to claim 8, wherein the ^(99m) Tc is derived from sodium pertechnetate.
 10. Method according to claim 9, wherein ^(99m) Tc is administered in an amount of about 0.2 mCi/kg to about 0.43 mCi/kg of the patient's body weight.
 11. Method according to claim 10, wherein labeling yield of red blood cells is at least about 90% at 45 minutes after ^(99m) Tc dosing.
 12. A method according to claim 7, wherein the stannous chloride and potassium perchlorate are in a weight ratio of about 1:20 to about 1:30.
 13. A method according to claim 7, wherein the stannous chloride and potassium perchlorate are in a weight ratio of about 1:24.
 14. A method of carrying out equilibrium blood pool imaging in a patient, said method comprisingorally administering to the patient about 8.9 mg/kg body weight to about 42 mg/kg body weight of a composition consisting essentially of (1) about 10 mg to about 30 mg stannous chloride, and (2) about 250 mg to about 1000 mg potassium perchlorate; incubating red blood cells in vivo in the presence of Sn²⁺ for a time sufficient for tin to be absorbed by red blood cells in the patient; contacting the resulting tinned red blood cells with ^(99m) Tc; incubating the tinned red blood cells with the ^(99m) Tc for a time sufficient to radiolabel the cells; and imaging a blood pool of the patient with means for detecting and reporting radioactivity emitted by the ^(99m) Tc.
 15. A kit for tinning red blood cells of a patient in vivo in preparation for radiolabeling the cells with ^(99m) Tc, wherein the kit contains a drug consisting essentially of(1) a solid, unit dose of stannous compound consisting essentially of about 10 mg to about 30 mg of stannous chloride sufficient to tin the red blood cells in vivo; and (2) a solid, unit dose of potassium perchlorate in an amount of about 250 mg to about 1000 mg sufficient to substantially inhibit absorption of ^(99m) Tc by the gut of a patient, while permitting the tinning of red blood cells and radiolabeling of red blood cells with ^(99m) Tc; wherein the stannous chloride and potassium perchlorate are in a weight ratio of about 1:5 to about 1:100.
 16. Kit according to claim 15, wherein each unit dose is in the form of a tablet or capsule.
 17. Kit according to claim 16, wherein the tablets or capsules are individually sealed in separate compartments in a package.
 18. Kit according to claim 17, wherein the dose of stannous chloride and the dose of potassium perchlorate are combined to form a single, solid dose.
 19. Kit according to claim 18, wherein the kit includes directions for use of the drug contained in the kit.
 20. A pill in the form of a tablet of capsule suitable for oral administration to a patient for tinning red blood cells of the patient in vivo in preparation for radiolabeling the red blood cells with ^(99m) Tc, wherein said pill comprises a composition consisting essentially of(1) about 10 mg to about 30 mg stannous chloride, and (2) about 250 mg to about 1000 mg potassium perchlorate.
 21. A pill according to claim 20, in which the stannous chloride is stannous chloride dihydrate.
 22. A pill according to claim 20, wherein said composition contains a pharmaceutically acceptable carrier.
 23. A pill according to claim 20, in which the stannous chloride and potassium perchlorate are in a weight ratio of about 1:24. 